This invention pertains generally to the field of electrical heart pacemakers and more specifically to pacemakers of the type in which pacing rate is regulated by means of a sensor responsive to the patient's need for cardiac output and to pacemakers adapted to operate in an atrial synchronous mode, generally referred to as "VDD" pacemakers.
VDD pacemakers are first disclosed in U.S. Pat. No, 3,648,707 issued to Wilson Greatbatch on Mar. 14, 1972. This pacemaker included electrodes for sensing contractions of the atrium and ventricle and a pulse generator for pacing the ventricle. After sensing a contraction of the ventricle or pacing the ventricle, a lower rate timer was restarted. If this timer expired, it triggered generation of a ventricular pacing pulse. Also included was an A-V interval timer, initiated in response to the sensing of an atrial contraction. On expiration of this timer, the ventricular pacing pulse would also be triggered. The pacemaker included a third timer, defining an upper rate interval initiated following ventricular pacing or sensing of a ventricular contraction. During the upper rate interval, time out of the A-V interval would not be effective to trigger a ventricular pacing pulse. This allowed for inhibition of the ventricular pulse generator in the event that a natural ventricular contraction followed a sensed atrial contraction. It also defined a maximum synchronous pacing rate, corresponding to the upper rate interval. If the atrial rate exceeded this rate, the pacing rate would fall to the higher of one-half of the sensed atrial rate or the rate determined by the lower rate timer.
An improvement to this pacemaker was set forth in U.S. Pat. No. 4,059,116 issued to Adams on Jan. 12, 1976, incorporated herein by reference in its entirety. In this pacemaker, rather than preventing generation of a ventricular stimulus in response to time out of the A-V interval during the upper rate interval, the stimulus was instead delayed until the expiration of the upper rate interval. In addition, a post-ventricular atrial refractory period was defined after each ventricular pacing pulse and each sensed ventricular contraction, during which an atrial contraction would not initiate timing of the A-V interval. The net result of these features was to produce a pacemaker which displayed an improved response to atrial contractions occurring at intervals less than the upper rate interval. The pacemaker in this case generated ventricular stimulation pulses separated by the upper rate interval, displaying gradually lengthening A-V intervals until an atrial contraction fell within the post-ventricular atrial refractory period. The pacemaker would resynchronize on the next subsequent atrial contraction, mimicking the natural condition known as Wenckebach behavior. In commercially marketed pacemakers employing the Adams invention, the behavior of the pacemaker in the presence of high natural atrial rates is referred to as "Pseudo-Wenckebach" upper rate behavior.
Recently, variation of pacing rate in response to sensed physiologic parameters, such as physical activity, oxygen saturation, blood pressure and respiration has been the subject of much research and development activity. U.S. Pat. No. 4,428,378 issued to Anderson et al discloses a pacemaker which varies its pacing rate in response to sensed physical activity. Pacemakers as described in this patent have been widely commercialized. A similar pacemaker, varying its pacing rate in response to variations in oxygen saturation, is disclosed in U.S. Pat. No. 4,467,807, issued to Bornzin.
Physiologic sensors have been integrated into dual chamber pacemakers, as illustrated in U.S. Pat. No. 4,890,617, issued to Markowitz et al on Jan. 2, 1990, incorporated herein by reference in its entirety. In this patent, a DDD type pacemaker having pacing intervals regulated by a sensor responsive to physical activity is disclosed. In this device, a fixed post-ventricular atrial refractory period is defined following each ventricular pacing pulse.
An additional dual chamber rate responsive pacemaker is disclosed in U.S. Pat. application Ser. No. 07/249,046 filed Sep. 23, 1988, by Barouh Berkovits for a "DUAL CHAMBER RATE RESPONSIVE PACEMAKER", also incorporated herein by reference in its entirety. In this pacemaker, atrial rate is monitored and, in response to an atrial rate in excess of a predetermined upper rate operation of the pacemaker is changed to desynchronize it from the atrium, with pacing rate controlled by a sensor of the patient's demand for cardiac output. This pacemaker also employed a fixed post-ventricular atrial refractory period.
An additional dual chamber, rate responsive pacemaker is described in the article entitled "Rate Responsive Dual Chamber Pacing", published in Pace, Vol. 9, Nov.-Dec. 1986, pp. 987-991. In this device, regulation of the pacing rate was determinable either by the natural atrial rate or the activity rate. The pacemaker was so configured that it would not follow atrial rates substantially in excess of the rate defined by the sensor. This was accomplished by extending the post-ventricular atrial refractory period in response to detection of an atrial rate substantially in excess of the sensor determined pacing rate.
Numerous patents have dealt with dual chamber pacemakers which vary their post-ventricular atrial refractory periods in an attempt to prevent generation of pacemaker mediated tachycardias (PMT's). One such pacemaker is disclosed in U.S. Pat. No. 4,920,965 issued May 1, 1990 for a "DUAL CHAMBER PACEMAKER WITH ADAPTIVE ATRIAL ESCAPE INTERVAL", incorporated herein by reference in its entirety. In this pacemaker, a post-ventricular atrial refractory interval is calculated based upon the time of occurrence of the atrial contraction, relative to the preceding ventricular pacing pulse or sensed ventricular contraction. This pacemaker gradually decreased its post-ventricular atrial refractory period in response to increasing natural atrial rates. Alternatively, it has been suggested to monitor the timing of atrial contractions with respect to previous ventricular contractions and if their timing indicates that the atrial contractions are probably retrograde P-waves, to extend the post-ventricular atrial refractory period beyond the measured time of occurrence of the retrograde P-waves. Such pacemakers are disclosed in U.S. Pat. Nos. 4,544,921 and 4,503,857, issued to Boute et al.